On essentially every method in the API it is possible to specify a
custom subset of fields on an object by specifying a fields
parameter.

There are two use cases that this functionality aims to serve:

First, if you are writing an application that loads a lot of data but
only uses some of it, specifying a limited subset of fields can reduce
response time and bandwidth. We’ve seen this approach be particuarly
useful for mobile applications where bandwidth is at a premium.

An example would be a legislator search with
fields=first_name,last_name,leg_id specified. All legislator objects
returned will only have the three fields that you requested.

Second, you can actually specify a set of fields that includes fields
excluded in the default response.

For instance, if you are conducting a bill search, it typically does not
include sponsors, though many sites may wish to use sponsor information
without making a request for the full bill (which is typically much
larger as it includes versions, votes, actions, etc.).

A bill search that specifies fields=bill_id,sponsors,title,chamber
will include the full sponsor listing in addition to the standard
bill_id, title and chamber fields.

You may notice that the fields documented are sometimes a subset of the
fields actually included in a response.

Many times as part of our scraping process we take in data that is
available for a given state and is either not available or does not have
an analog in other states. Instead of artificially limiting the data we
provide to the smallest common subset we make this extra data available.

To make it clear which fields can be relied upon and which are perhaps
specific to a state or subset of states we prefix non-standard fields
with a +.

If you are using the API to get data for multiple states, it is best to
restrict your usage to the fields documented here. If you are only
interested in data for a small subset of our available states it might
make sense to take a more in depth look at the API responses for the
state in question to see what extra data we are able to provide.